This invention relates to an image processing method and apparatus for processing image data.
In an image processing method known in the art, the number of gray levels of each color in a color image is transformed (by a level-number transformation) to two or a prescribed number while half tones are maintained in a half-tone image output of an image recording apparatus or the like. Generally, the error-diffusion method or a mean-error minimization method, which is substantially the same as the error-diffusion method, is employed as the method of transforming the number of gray levels of image data. These methods, which are applied to various image processing apparatus as binarizing methods for preserving half-tone density, execute the level-number transformation while a difference in density (a transformation error), with respect to the original pixel data, that occurs when each pixel is subjected to the level-number transformation is distributed to as yet unprocessed pixel data and accumulated.
However, in a case where the number of levels of original image data containing random noise is transformed by the above-mentioned level-number transformation method, the random noise becomes mixed with the transformation error and is thus distributed to the still unprocessed pixels, where the noise is accumulated as a result. Consequently, random noise increases and invites a marked decline in picture quality.